The magnetic disk drive comprises a magnetic head to read and write data on a recording medium. To raise the recording capacity per unit area of the magnetic disk, it is necessary to raise the areal recording density. In the conventional longitudinal recording system, however, if the recorded bit length is short, it is not possible to raise the areal recording density due to thermal fluctuations of the medium in magnetization. A solution to this problem is the perpendicular recording system which perpendicularly records a magnetization signal on the medium. For perpendicular recording, the GMR (Giant Magneto-Resistive) head, the TMR (Tunneling giant Magneto-Resistive) head providing further higher read output and the CPP (Current Perpendicular to the Plane) type GMR head which passes current perpendicularly to the plane of the film can be used as well.
In a perpendicular recording system, a double-layered recording medium having a soft magnetic underlayer called a SLU (Soft Under Layer) is used with a single-pole write head having a main magnetic pole piece and an auxiliary magnetic pole piece. The presence of the SLU in the perpendicular recording system raises the writing ability of the head. Since the head can therefore produce a recording magnetic field beyond 10 T (teslas), it is possible to use such a highly coercive recording medium as to have a coercive force of 6 kOe or higher. Although perpendicular recording provides such high writability, however, it is pointed out that external magnetic fields concentrate in the main magnetic pole piece and may erase a signal written on the medium. For example, Patent Document 1 (Japanese Patent Laid-Open No. 2003-123210) discloses a technique to prevent external magnetic filed from entering the main magnetic pole piece by incorporating a shield.
In order to make higher the recording magnetic field, the above-mentioned main magnetic pole piece is made of some of Fe- or Co-based metals having high saturation magnetic flux densities (Bs). Although their saturation flux densities Bs are high, these materials are apt to corrode due to poor corrosion resistance.
Disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2003-77105) is a configuration which allows the use of such a high saturation flux density metal. To prevent its corrosion during processing in an aqueous solution, an external terminal is provided which is connected to the lower magnetic pole piece. The standard electrode potential of this terminal is higher than that of the magnetic film and its surface area is larger than the cross-sectional area of the magnetic film on the air bearing surface facing the magnetic recording medium. In addition, it is disclosed that an internal metal film made of Cu is used to electrically connect the lower magnetic pole piece and the external terminal. The lower magnetic pole piece is extended to a section where it is connected with the upper magnetic pole piece and to a section above which the coil is connected with wires leading to the recording terminals.